“It has been an unexpected journey indeed, and in the words of Professor Tolkien himself, ‘a tale that grew in the telling,’ ” wrote Mr. Jackson, who said he will use the three-movie format to “tell more of the tale.”

“The Hobbit: An Unexpected Journey,” which is the first of what will now be three films based on “The Hobbit,” by J.R.R. Tolkien, remains set for release by Warner Brothers and its New Line Cinema unit, along with Metro-Goldwyn-Mayer, on Dec. 14.

The second film is currently scheduled for release in December 2013. The third film can be expected in the summer of 2014, Warner said in a statement.

In an appearance earlier this month at the Comic-Con International fan convention, Mr. Jackson explained that some material from the related “Lord of the Rings” cycle, and a small bit of invention that added a stronger feminine presence to Mr. Tolkien’s Hobbit story, had expanded its cinematic potential.